Since I use iTerm and always have at least one window open what this does is to tell iTerm to spawn a new window with a bash shell and then execute IRB, requiring my code.

The net result is that hitting Cmd+Ctrl+R gives me my code in an IRB session ready to test via the REPL. I can poke around, hit Ctrl+D to get rid of the IRB session, go back to TextMate & make some changes, then hit Cmd+Ctrl+R to get a new IRB with the updated version ready to test.